1. Field of the Invention
The present invention relates to effecting a reliable electrical contact in a semiconductor device between a metalized wiring trace and a silicon substrate. More particularly the present invention pertains to a diffusion barrier for preventing the migration of silicon from the semiconductor substrate into the metalized electrical contact.
2. Background Art
Where a metalized wiring trace, such as a wiring trace of aluminum makes contact with the surface of a silicon substrate, silicon migration is common from the substrate into the metalized wiring trace under certain biasing conditions. This movement of silicon tends to displace aluminum at other parts of the associated semiconductor device. It also tends to erode the surface of the substrate, causing pitting at the contact site. Pitting of prolonged duration tends to produce deep fissures which can jeopardize the integrity of the PN junction at the edge of the conductivity well in which the electrical contact is being effected. This latter problem becomes particularly acute as the trend toward miniaturization and densification calls for shallower conductivity wells.
Thus, in the fabrication of semiconductor devices, structures must be included which prevent the migration of material from the semiconductor substrate into metalized electrical contacts through the interface at which such contact is effected. One solution has been to make metalized contacts out of aluminum saturated with silicon. This inhibits migration at the contact interface, but requires the use of relatively exotic metalization materials.
Alternatively, efforts have been undertaken to construct a barrier to silicon migration at the contact interface. In this approach, titanium nitride (TiN) has been found to offer promise. In layer form disposed between a silicon substrate and a metalized wiring contact, titanium nitride affords an acceptably low-resistance electrical coupling between the two materials on either side, while also functioning to retard the migration of silicon into the metalized layer. Typically, such diffusion barriers have either been deposited as titanium nitride directly on the surface of the substrate, or developed by annealing in an atmosphere of nitrogen a layer of titanium that has been previously deposited on the surface.
The barrier layers of titanium nitride need to be uniformly of a minimal thickness, if the barrier function is to be successful. While breaks in the barrier layer are obvious structural failures, it is not sufficient to effect mere continuity of the layer in order to achieve a satisfactory barrier. Any portions of the barrier layer that are overly thin will ultimately fail to prevent silicon migration, and thus be the cause of pitting and other migration-related problems.
The task of creating a diffusion barrier which at all locations exhibits at least the requisite minimum thickness has become increasingly difficult as the drive toward miniaturization and device densification continues in the field of semiconductor devices. Contacts between metalized wiring traces and the surfaces of a silicon substrate are in most instances effected by way of a contact well etched through an electrically insulating layer of silicon dioxide on the surface of the substrate. One consequence of miniaturization and device densification is that less space is available to be used for such contact wells. Thus, the trend is to make them smaller. Nevertheless, as the diameter of such contact wells decreases, the deposition of material into the well from which to form a diffusion barrier becomes increasingly difficult.
This is due to the step coverage pattern observed to occur in deposition efforts at such wells. The material being deposited simply finds its way less readily into the contact well, rather than onto the top surface of the surrounding insulative layer. Even the deposited material that does enter the well has about as much likelihood of ending up on the walls of the contact well as on its floor. It is only on the floor, however, that the deposited material can truly be effective in producing a diffusion barrier.
Typically, the step coverage pattern that results is characterized by a very then layer of the material at the bottom of the contact well even when a thick layer is produced on the top of the surrounding insulative layer. In the typical step coverage pattern, an overhang structure made of the deposited material develops on the top of the sides of the contact well near its opening. The overhang structure is created from material which otherwise should have been deposited on the floor of the contact well. The overhang also closes the opening to the contact well, in effect shadowing the well floor from the deposition of additional material. The problem of thin depositions on the floor of a contact well is particularly acute at the periphery of the floor near the corners between the floor and the walls.
Poor thin floor layering and corner coverage begin to be dominating characteristics in contact wells having the combination of depths in the range of from about 1.00 to about 2.00 microns and widths in the range of from about 0.70 microns to about 1.00 microns. Under such circumstances, the thickness of the floor layer will be only approximately 30 percent to approximately 50 percent of the thickness of the layer on the top surfaces of the surrounding insulation layer. In addition, overhang structures on the sides of the well cause the floor layer to be even thinner at its periphery.
The problem of the inadequate deposition of material into a contact well cannot be resolved by simply depositing thicker layers of that material over the entire substrate. The materials deposited in creating diffusion barriers, namely, pure titanium (Ti) and titanium nitride (TiN) are not particularly good electrical conductors. Thick accumulations of such materials on the surface of a substrate will require in turn that the metalized wiring traces be correspondingly thicker if standard specification requirements for low wiring trace conductivity are to be complied with.
Concrete examples of these problems will be appreciated through a discussion of two known, but flawed, methods of creating diffusion barriers of titanium nitride.
FIG. 1 illustrates a typical semiconductor silicon substrate 10, having formed at the surface 12 thereof a P-type conductivity well 14 having a lower boundary 16 comprising a PN junction with the balance of the material of substrate 10. Formed on surface 12 of substrate 10 is a relatively thick electrically nonconductive insulative layer 18, typically comprised of silicon dioxide (SiO.sub.2). In order to effect electrical coupling with substrate 10, it is necessary to form a contact well through insulative layer 18 to surface 12.
The commencement of this process is illustrated in FIG. 1 by the formation on insulative layer 18 of a patterned resist mask 20 having an opening 22 developed therein at a position corresponding to the desired location for an electrical contact with substrate 10. The structure in FIG. 1 is then subjected to a controlled, dry anisotropic etching, typically in a plasma of carbon tetrafluoride (CF.sub.4).
When etching through insulative layer 18 is completed, photo-resist mask 20 is removed, resulting in the structure illustrated in FIG. 2A. There a contact well 24 can be seen to have been formed through insulative layer 18 so as to have a floor 26 and walls 28. As the purpose of contact well 24 is to permit electrical contact to be made with surface 12 of substrate 10, floor 26 of contact well 24 defines a contact surface on substrate 10. Before the surface of the structure shown in FIG. 2A is metalized, however, it is necessary to produce on floor 26 of contact well 24 a barrier to the migration of silicon from substrate 10 into such a metalized wiring trace.
A first known method for producing such a diffusion barrier is illustrated in the sequence of FIGS. 2A, 2B, 2C and 2D. A clearer understanding of the process and problems involved will be gained by reference also to the enlarged detail views appearing in FIGS. 3A, 3B, and 3C of the corners 30 at the outer periphery of floor 26 of contact well 24.
The walls 28 of contact well 24 are etched to remove therefrom polymers deposited during the dry etch in carbon tetrafluoride. Then a first layer 32 of titanium is formed by sputtered deposition on floor 26 and walls 28 of contact well 24, as well as on the top surface 33 of insulative layer 18. This is accomplished by placing semiconductor substrate 10 in a semiconductor processing chamber at low pressure and biasing substrate 10 as a cathode relative to a target anode a of titanium. Argon introduced into the pressure chamber is ionized to produce a plasma. The plasma impacts the titanium target, freeing ions thereof into the rarified gas of the processing chamber. The ions of titanium are driven by the electrical bias established between substrate 10 and the titanium target toward substrate 10, accumulating on the surface thereof as first layer 32.
At this point the step coverage phenomenon becomes relevant for the first time. In FIG. 2A the contact well 24 depicted is intended to be a contact well of relatively small dimension. Contact well 24 has a diameter in the range of from about 0.70 to about 1.00 microns and a relatively high aspect ratio of about 1 (1.00), which results in a depth of from about 1.00 to about 2.00 microns. As a result, the disposition of first layer 32 of titanium in contact well 24 is not uniform. Atoms of titanium moving towards floor 26 of contact well 24 are in many instances drawn onto walls 28 instead. This results in the depositions of titanium on floor 26 of contact well 24 assuming the form of a relatively thin portion 34 of first layer 32. Thin portion 34 is thickest at the center of floor 26, but tapers toward corner 30 of contact well 24. This thinning is partially a result of the diversion onto walls 28 of material otherwise destined to accumulate on floor 26 of contact well 24. The accumulation of titanium onto walls 28, however, also forms overhang portions 36. These tend to shadow corners 30 from subsequent deposition and to thereby increase the thinning of depositions on floor 26 at its periphery. The resultant coverage at corner 30 of contact well 24 by first layer 32 of titanium is relatively unsatisfactory, as shown with additional detail in FIG. 3A.
Thereafter, the structure shown in FIG. 2B is annealed at a high thermal temperature in ambient nitrogen. In this manner, first layer 32 of titanium is progressively converted from its exposed surface into a strata 38 of titanium nitride (TiN). Titanium does not react during annealing with insulative layer 18. Thus, the portions of first layer 32 of titanium on the top surface of insulative layer 18 and on walls 28 of contact well 24 are able to be fully converted to titanium nitride.
The titanium nitride layer produced exhibits a volume somewhat enlarged from that of the original titanium layer 32. Thus, strata 38 (FIG. 2C) of titanium nitride is thicker proportionately than original layer 32, both on the top surface 33 of insulative layer 18 and on walls 28 of contact well 24.
While titanium nitride functions very desirably as a barrier to the diffusion of silicon from substrate 10 into the metalized contact that will eventually be placed in contact well 24, the thickest portions of strata 38 of titanium nitride are not formed in the areas in which electrical coupling is to be effected with substrate 10. These thicker portions include upper layer 40 on the top surface of insulative layer 18 and overhang portions 42 on walls 28 of contact well 24. Disadvantageously, overhang portion 42 tapers into an extremely thin structure at corner 30 of contact well 24. On the very floor 26 of contact well 24, the formation of titanium nitride is particularly unsatisfactory, resulting in a thin portion 44 of strata 38 of titanium nitride.
The reason that the layer of titanium nitride in the floor 26 of contact well 24 is so minimal relates to the interaction between thin portion 34 of first layer 32 of titanium shown in FIG. 2B with the material of substrate 10 upon which thin portion 34 is originally disposed. During annealing, while the exposed surface of thin portion 34 is reacting with free nitrogen to form titanium nitride, the material of thin portion 34 adjacent to substrate 10 is induced by the heat of annealing to migrate across surface 12 thereof into the lattice of silicon substrate 10, forming a diffusion region 46 (FIG. 2C) of titanium silicide (TiSi.sub.2). As this process occurs more rapidly than the conversion of titanium into titanium nitride at the surface of thin portion 34, most of the titanium in thin portion 34 is consumed in producing diffusion region 46. Only a small surface fraction of thin portion 34 of first layer 32 of titanium is available for conversion into the migration barrier material, titanium nitride, in thin portion 34 of strata 38.
Thus, at floor 26 of contact well 24, a dual conversion process occurs relative to the titanium of first layer 32 thereupon. This process will be discussed with additional clarity in relation to FIG. 3A, which is a schematic, detailed view of corner 30 of contact well 24 illustrating the effects on the mass of thin portion 34 of first layer 32 of titanium from the formation of diffusion region 46 alone. It should be understood that at the same time as diffusion region 46 is being formed from the mass of thin portion 34 of first layer 32 the balance of thin portion 34 is being converted into titanium nitride. In order to enhance a clear understanding of the dual processes involved, reference is made to FIG. 3B.
There the surface 48 of the original profile of thin portion 34 of first layer 32 of titanium is illustrated for comparison by a dashed boundary. The migration of titanium from what was thin portion 34 of first layer 32 into substrate 10 results in the formation of diffusion region 46 of titanium silicate. That process, however, causes expansion in the silicon involved, raising the former floor 26 of contact well 24 in a domed surface 50 elevated even in relation to the former top surface 48 of thin portion 34 of first layer 32.
It can further be observed in FIG. 3B that most of the mass of former thin portion 34 has been consumed in this process, leaving therefrom only a surface layer 52 of titanium for reacting with ambient nitrogen in the annealing process to produce a barrier layer of titanium nitrite. Overhang portion 36 of first layer 32 of titanium is shown in FIG. 3B as substantially unchanged, as titanium does not migrate into insulative layer 18. Accordingly, the full mass of overhang portion 36 remains in place in order to participate in conversion during the annealing process into titanium nitride. Nevertheless, as has already been pointed out earlier, the portion of the diffusion barrier formed on walls 28 of contact well 24 contributes to the barrier function only at the very bottom of walls 28 at corner 30 of contact well 24. There, the amount of titanium remaining from the formation of diffusion region 46 is even less than the thickness of surface layer 52 toward the center of contact well 24.
FIG. 3C illustrates the effect of the second portion of the process occurring during annealing in which titanium in surface layer 52 and in overhang portion 36 of first layer 32 of titanium are converted into titanium nitride. In the process, the volume of the corresponding material is enlarged. Accordingly, in FIG. 3C for comparative purposes, the outer surface 54 of overhang portion 36 and the upper surface 56 of surface layer 52, both of titanium, are indicated by dashed lines. In place of each, respectively, appear overhang portion 42 and thin portion 44 of strata 38 of titanium nitride. The resultant upward expansion of surface layer 52 while minimal and the lateral expansion of overhang portion 42 at corner 30 of contact well 24 serves to thicken the layer of titanium nitride in the immediate area in which electrical coupling with substrate 10 is actually effected. Nevertheless, because of the rapidity of the migration of titanium from thin portion 34 of first layer 32 relative to the conversion of titanium in thin portion 34 into titanium nitride, the effectiveness of the resultant diffusion barrier is not reliable.
In an effort to increase the amount of titanium in surface layer 52 which is available for conversion into titanium nitride, the deposition of additional quantities of titanium on floor 26 of contact well 24 have been attempted. Two problems arise as a result first, the success of depositing titanium on floor 26 of contact well 24 is, however, reflected in extremely thick layers of titanium on the top surface 33 of insulative layer 18, which in turn calls for an increase in the thickness of the metalized layer deposited thereupon in forming a metalized lead 58 shown in FIGS. 2C and 3C in contact well 24 engaging strata 38 of titanium nitride.
Second while a diffusion region of titanium silicide, such as diffusion region 46, is desirable, diffusion region 46 need only be relatively shallow in order to accomplish its purpose of enhancing the conductivity of the interface effected at the contact surface defined by floor 26 of contact well 24. When additional titanium is deposited on floor 26, although some of the increase will be converted into titanium nitride, additional quantities of the increase just migrate into substrate 10, causing a deepening of diffusion region 46 thereinto. Diffusion region 46 thus extends closely to boundary region 16 of conductivity well 14 which has been known to produce shorting through conductivity well 14 into the portion of substrate 10 below boundary 16.
The method illustrated in FIGS. 2A through 2D and in FIGS. 3A through 3C thus has the dual drawback of producing an overly deep diffusion region of titanium silicide or in lieu thereof an overly thin barrier of titanium nitride on floor 26 of contact well 24 where the barrier to silicon migration is most essential. This problem is most critical in corners 30 of contact well 24, where the shadowing effect of overhang portion 36 of first layer 32 of titanium is most pronounced due to step coverage patterns of sputter deposition. Efforts to overcome one of these two flaws in the resultant electrical contact work against the other. Ameliorating one intensifies the other, and the method disclosed cannot ultimately be substantially improved.
Accordingly, resort has been made in the art to a second known method for producing a diffusion barrier, which is illustrated and will be discussed in relation to the series of steps depicted in FIGS. 4A through 4E in combination with the detailed view of FIG. 5. The second method, instead of relying upon the annealing process to produce the requisite layer of titanium nitride, deposits that layer utilizing reactive sputter deposition in an atmosphere of nitrogen. This modification then frees the semiconductor manufacturer from the need to place excessive titanium in the contact well in order that some of the titanium escape migration into the substrate to form a titanium silicate diffusion region. Accordingly, the titanium layer deposited in the first instance in the prior art method already described can be reduced in thickness, both on the floor of the contact well and on the upper surfaces of the insulative layer thereabout. The latter consequence then permits the use of thinner metalized wiring traces than are possible under standard conductivity specifications utilizing the method already described.
In FIG. 4A in the second method, a contact well 24 identical to that illustrated in FIG. 2A is formed through insulative layer 18 to surface 12 thereof. Contact well 24 has a floor 26 at surface 12 which defines the contact surface at which electrical coupling with substrate 10 is intended, walls 28, and corners 30. A first layer 32a of titanium is produced on top surface 33 of insulative layer 18, and floor 26 and walls 28 of contact well 24. In the second method, first layer 32a of titanium is not relied upon for the production of the ultimate barrier layer of titanium nitride. Accordingly, as can be appreciated by comparison, first layer 32a of titanium can be substantially thinner than first layer 32 of titanium shown in FIG. 2B. Nevertheless, because of the aspect ratio and the small diameter of contact well 24, first layer 32a of titanium is reduced on floor 26 of contact well 24 to a thin portion 34a, while on walls 28 of contact well 24 overhang portions 36a still tend to further shadow the development of coverage by titanium in corners 30 of contact well 24.
Before annealing the structure shown in FIG. 4B in order to develop from thin portion 34a thereof a diffusion region of titanium silicate, a first layer 60 (FIG. 4C) of titanium nitride is deposited on first layer 32a of titanium. This is accomplished in the manner of the sputter deposition of titanium, except that the process is conducted in an atmosphere of nitrogen, rather than argon.
In relation to layer 60, however, step deposition patterning is also apparent. Layer 60 of titanium nitride thus comprises a thin portion 62 at floor 26 of contact well 24 and overhang portions 64 on each wall 28 thereof. As layer 60 of titanium nitride is deposited over another layer, such as first layer 32a of titanium, which itself possesses an overhang portion, the ability to place material of layer 60 of titanium nitride at the bottom of contact well 24 and to insure the integrity of that layer at the corners 30 of contact well 24 is more difficult. Significantly, however, thin portion 62 of layer 60 of titanium nitride constitutes the totality of the structure by which migration of silicon across the contact surface is precluded.
Thereafter as shown in FIG. 4D, the structure of 4C is subjected to an annealing heat treatment. This treatment does not alter the material of layer 60 of titanium nitride, but only permits titanium in thin portion 34a of first layer 32a of titanium to migrate into substrate 10 producing diffusion region 46a of titanium silicide. This process increases the volume of material below thin portion 62 of layer 60 of titanium nitride lifting floor 26 of contact well 24 into a domed surface 50a upon which thin portion 62 of layer 60 of titanium nitride is lifted and stretched slightly in its lateral dimension.
Thereafter as shown in FIG. 4E, a metalized lead 58 is deposited filling contact well 24 and engaging layer 60 of titanium nitride. The diffusion barrier resulting includes layer 60 of titanium nitride in combination with a relatively shallow diffusion region 46a.
As can be seen in additional detail in the enlarged view appearing in FIG. 5, diffusion region 46a of titanium silicide is advantageously shallower than diffusion region 46 shown in FIG. 3C. The shallower penetration into substrate 10 by diffusion region 46a reduces the likelihood of shorting across boundary 16 of conductivity well 14. Nevertheless, even in the deposition of the material of layer 60 of titanium nitride in the bottom of contact well 24, it is difficult to produce an adequately thick diffusion barrier at that location without resorting to excessive depositions of titanium nitride on the top surface 33 of insulative layers 18. Frequently the combined thickness of first layer 32a of titanium and layer 60 of titanium nitride at that location is at least as great as the single strata 38 of titanium nitride produced in the first prior art method and shown in FIGS. 2C and 2D. This accordingly places similar constraints on efforts to thicken the actual diffusion barrier produced as were encountered in that earlier described prior art method.
The problems of breaches in that diffusion barrier continue in the second method to be most acute at corners 30 of contact well 24. The results is an unreliable diffusion barrier and continued problems of pitting which appear to be able to be overcome only by increasing the quantity of titanium and titanium nitride disposed on the top of the surrounding insulative layers. This in return is reflected in undesirably thick metalized wiring traces.